Optical communications using optical fibers is presently a rapidly developing field due to its high data transmission rate at relatively low cost. Early optical transmission systems could not span long distances before the losses in the fibers reached a level that required that the signals be detected and regenerated in a repeater. Simpler systems began to evolve when optical amplifiers were developed; the optical amplifiers are simpler than were the repeaters. The optical amplifier of choice is presently a rare earth doped optical fiber. A commonly used rare earth dopant is erbium and the erbium doped fiber amplifier is often referred to by the acronym EDFA. Optical amplifiers are replacing repeaters in long distance transmission systems. Optical amplifiers are also useful with star couplers because they can be used to replace lost signal power.
Capacity is frequently a critical parameter for system viability, and bidirectional systems are desirable when the increased capacity or other attributes afforded by the bidirectional fiber is required. An exemplary bidirectional system is described by Murphy et al(Murphy) in Journal of Lightwave Technology, 6, pp. 937-945, June 1988. Murphy used LiNbO.sub.3 devices as external modulators for lasers. The signals were combined and transmitted on a single fiber. A star coupler useful with fiber amplifiers is described in U.S. Pat. No. 5,140,655 issued on Aug. 22, 1992 to Ernest Bergmann. However, the couplers specifically disclosed do not optically isolate the laser and for many purposes, the laser used as a light source should be isolated from reflections. Bidirectional fiber amplifiers are discussed by Barnard et al(Barnard) in IEEE Photonics Technology Letters, 4, pp. 911-914, August 1992. Of particular interest is the configuration depicted in Barnard's FIG. 2. This is a bidirectional fiber amplifier which uses two four port optical circulators for signal separation. Barnard's technique separates the counter propagating signals, amplifies the signals, and then recombines the signals. See, also, Cheng, page 69, Optical Fiber Communication, 1992, for a description of a bidirectional system. FIG. 3(b) shows the bidirectional system. Two tour port optical circulators are also used in this configuration. See, also, Sato et al., IEEE Transactions Photonics Technology Letters, 3, pp. 1001-1003, November 1991; Farre, et al, IEEE Photonics Technology Letters, 4, pp. 425-427, April 1993; and Guo et al, IEEE Photonics Technology Letters 5, pp. 232-235, February 1993, for descriptions of other bidirectional systems.